1. Field of the Invention
This invention relates to improved phenolic foam having a uniform closed-cell structure with cells substantially free of both ruptures and perforations. The phenolic foam has improved thermal insulation properties and compressive strength. The invention also relates to a method for preparing the improved phenolic foam by curing and foaming a foamable phenolic resole composition with certain anhydrous aryl sulfonic acid catalysts in a substantially closed mold under restraining pressures of at least about 3 psi above atmospheric pressure. The invention also relates to an aqueous phenol formaldehyde resole for preparing the improved phenolic resole foamable composition and phenolic foams.
2. Prior Art
Phenolic foams prepared from phenolic resoles have been known for years. It is generally agreed that phenolic foams have the best fire rating of any known foam insulation. Phenolic foam does not burn even when contacted by the flame of a blow torch and gives off minimal amounts of toxic gases. Phenolic foams can stand temperatures of 375.degree. F. without serious degradation. Phenolic foams have an ASTM E-84 Steiner Tunnel Flame Spread Rating around 5, a Fuel Contribution of about 0 and a Smoke Rating of about 5.
Despite these advantages and generally favorable economics, phenolic foams have not penetrated the thermal insulation market. The reason phenolic foams have not been successful is that phenolic foams made heretofore have exhibited either an unsatisfactory initial thermal conductivity or an undesirable increase in thermal conductivity over time. Additionally, the compressive strength of prior art phenolic foams is not as high as desirable for normal handling. It has also been reported that prior art phenolic foams have serious problems with friability and punking.
The general composition and method for preparing phenolic foam are well known. Generally, a foamable phenolic resole composition is prepared by admixing aqueous phenolic resole, blowing agent, surfactant, optional additives and an acid curing agent into a substantially uniform composition. The curing catalyst is added in amounts sufficient to initiate the curing reaction which is highly exothermic. The exotherm of the curing reaction vaporizes and expands the blowing agent thereby foaming the composition. The foaming process is preferably performed in a substantially closed mold.
The general method for the continuous manufacture of phenolic foam insulation board is as follows. The foamable phenolic resole composition is prepared by continuously feeding into a suitable mixing device the aqueous phenolic resole, blowing agent, surfactant, optional additives, and acid curing catalyst. The ratio of these ingredients is varied depending on the density, thickness, etc. desired in the final product. The mixing device combines these ingredients into a substantially uniform composition which is continuously applied evenly onto a moving substrate, usually a protective covering such as cardboard, which adheres to the foam. The foaming composition is usually covered with another protective covering such as cardboard which becomes adhered to the phenolic foam. The covered foaming composition is then passed into a double belt press type apparatus where the curing exotherm continues to vaporize and expand the blowing agent, thereby foaming the composition as it is cured.
As mentioned, one of the main drawbacks of prior art phenolic foam is an unsatisfactory initial thermal conductivity (k value). It is believed that one of the main causes of phenolic foam having a poor initial thermal conductivity is due to the rupturing of the cell walls during the foaming and early curing of the foamable phenolic resole composition. This rupturing causes an immediate loss of fluorocarbon blowing agent which results in a poor initial thermal conductivity. Ruptured cell walls also provide ready passage of water into the foam, causing a further increase in thermal conductivity. It is also believed that ruptured cell walls deleteriously affect the compressive strength and other properties of the phenolic foams. Another main cause of initial poor thermal conductivity in phenolic foams is the loss of fluorocarbon blowing agent before the cell walls of the foaming compositions are sufficiently formed to entrap the blowing agent.
Also as mentioned, another drawback of prior art phenolic foams is the undesirable increase of thermal conductivity over time (k factor drift). Even in the prior art foams which have cell walls which are not ruptured and which have the fluorocarbon entrapped in the cells, the phenolic foams have a tendency to lose the fluorocarbon blowing agent over time with a corresponding increase in thermal conductivity. It is believed that there are two main causes of the increase in thermal conductivity over time. The first and main cause is the presence of small perforations or pinholes in the cell walls. These small perforations allow the fluorocarbon blowing agent to diffuse out over time and be replaced by air. This slow replacement by air causes an increase in thermal conductivity and loss of thermal insulation value. The small perforations also allow the phenolic foam to absorb water, thereby further increasing the thermal conductivity.
The other main cause of the loss of thermal conductivity over time is cracking of the cell walls. In many prior art phenolic foams the cell walls are very thin. When phenolic foams having thin cell walls are subjected to high temperatures, the cell walls dry out and crack. Also, since thermal insulation is normally subject to heating and cooling cycles with concomitant expansion and contractions, the cracking of the thin cell walls is aggravated. Cracking of the cell walls allows the fluorocarbon blowing agent to leak out over time with an and increase in thermal conductivity and loss of thermal insulation values.
The art has proposed several methods for overcoming the problem of poor thermal conductivity. For example, one method involves a two-step process comprising foaming the foamable phenolic resole composition initially under a vacuum followed by curing at high temperatures and low pressures. This method does produce a foam having a substantial number of cell walls which are not ruptured; however, there are still many cell walls which are either ruptured or which contain perforations or which are thin and readily crack when subjected to thermal stress. This method is also not commercially desirable because of the equipment that is necessary and the extended time that is required. Another method involves foaming and curing the foamable phenolic resole at low temperatures (i.e., less than 150.degree. F.). This method also reduces the number of cell walls that are ruptured but the resulting phenolic foam still has thin cell walls and perforations. Another method covered by a copending application assigned to the same assignee covers a method of foaming and curing the foamable phenolic resin composition while maintaining pressure on the foaming and curing composition. This method greatly reduces the number of ruptured cell walls but the resultant phenolic foam may still have a substantial number of ruptured cell walls or may have lost the blowing agent before the cell walls were cured and the cell walls may be thin and have perforations.
Other attempts at improving the thermal conductivity of phenolic foams have been based on developing specially modified phenolic resoles, or surfactants, or the use of certain additives in the foamable phenolic resole composition. None of these methods has been commercially successful. See, for example, U.S. Pat. Nos. D'Allesandro 3,389,094; Bunclark et al. 3,821,337; Moss et al. 3,968,300; Moss 3,876,620; Papa 4,033,910; Beal et al. 4,133,931; Bruning et al. 3,885,010; and Gusmer 4,303,758. Accordingly, it is the object of the present invention to provide an improved closed cell phenolic foam having cell walls without ruptures or perforations.
Another object of this invention is to provide an improved closed cell phenolic foam having low initial conductivity with little, if any, increase in the thermal conductivity over time without adversely affecting the friability, compressive strength or flammability characteristics of the phenolic foam.
A still further object of the present invention is to provide a composition and method for making the improved phenolic foams.
Additional objects and advantages of the present invention will be apparent to those skilled in the art by reference to the following description and drawings.